An effective quantitative method for the evaluation of endocrine-disrupting chemicals based on the secondary sexual characteristics of medaka (Oryzias latipes) using an S-rR strain has been developed in this study. Although the major characteristics of gross morphologies of fish for the detection of the effects of endocrine-disrupting chemicals are GSI (gonadosomatic index) and gonadal morphology, some disadvantages, such as low sensitivity, have been reported. The method we propose here involves the simple counting of the number of nodes in the second ray of the last anal fin. The S-rR strain was exposed to 17β-estradiol (E2) at three concentrations (16, 32 and 64 ng·l-1) from the fertilized-egg stage to the 70-day posthatch. At the termination of exposure, the number of nodes as well as the maximum number of small papillary processes on a ray and the number of rays with small papillary processes in the anal fin were determined. We found that the number of the ray nodes would satisfied with the purpose for quantitatively evaluating effects of endocrine-disrupting chemicals. The present study provides a more convenient and cost-effective method based on the secondary sexual characteristics of medaka that may be used for the reliable evaluation of the effects of endocrine disruptors.
*1 The Node Count Method is a new technique for quantitative evaluation of endocrine-disrupting chemicals based on the secondary sexual characteristics of medaka (Oryzias latipes) strain described in this paper. This technique was developed in the research that the National Institute of Advanced Industrial Science and Technology (AIST) entrusted to Sumika Technoservice Corporation.
